A 20 by 20 foot pond filled to 6 inches holds about 1,500 gallons, roughly 12,800 lbs of seawater, which contains at most about 350 lbs of sodium chloride, and at the parent article’s rate of a quarter inch a day it takes about 24 days to lose that depth. Stage the ponds in series so gypsum drops out early and common salt crystallises in a small last pond, and treat rain as the main enemy, since an inch of rain undoes about four days of evaporation.
Part of Salt Production & Sourcing
A solar evaporation pond is a shallow, sealed basin in which sun and wind evaporate seawater in stages until salt crystallises, built above the high-tide line.
Recompute The Parent’s Pond
The parent describes a pond 20 by 20 feet and 6 inches deep as holding “about 1,800 gallons (~15,000 lbs of seawater)” and yielding roughly 525 lbs of salt. Work it from the dimensions:
- Volume: 20 × 20 × 0.5 ft = 200 ft³
- Gallons: 200 × 7.48 gallons per ft³ = 1,496, about 1,500 (not 1,800)
- Mass of seawater: 200 ft³ × about 64 lbs per ft³ (seawater is about 1.025 times as dense as water; recalled) = about 12,800 lbs (not 15,000)
- All dissolved salts at 3.5%: 12,800 × 0.035 = 448 lbs
- Sodium chloride at 2.74% (27.4 g per kg; see Seawater evaporation): 12,800 × 0.0274 = 351 lbs
So a full pond holds up to about 350 lbs of common salt, not 525, and no pond recovers all of it; some stays in the bitterns. The parent’s evaporation arithmetic is sound on its own terms: 6 inches ÷ 0.25 inch per day = 24 days. The quarter inch a day is the parent’s assumption for a sunny climate and I cannot source it; measure your own rate with a marked stick in a test pan.
Why The Last Pond Is Small
Common salt does not crystallise until about 92% of the water has gone (Seawater evaporation). Measured as depth, a 6-inch pond is down to about 6 × 0.08 = 0.48 inch when salt starts to form. That is too shallow to work in. In practice the brine is moved to a smaller crystallising pond at some earlier point. As an illustration: if the brine is moved when 8% of the original volume remains, the crystalliser needs only 8% of the first pond’s volume. At the same depth that is 8% of the area; at one third the depth (2 inches) it is 24% of the area, from 0.08 × 6 ÷ 2.
The parent’s “3-4 ponds in series, first the largest” is this principle: evaporate the bulk in a large shallow pond, drop out gypsum in the early ponds, and finish in a small one where the salt forms and can be raked.
Rain, Wind And The Season
At the parent’s quarter inch a day, one inch of rain sets back the pond by 1 ÷ 0.25 = 4 days and dilutes the brine you have concentrated. A rainy spell on a nearly finished pond can undo weeks of work. Plan the season for the dry months, harvest before the rains, and have a way to drain rain off or cover the last ponds. Wind raises the rate; a still, humid day lowers it.
What You Need
- A site above high tide, level, with clay available (Pond Sealing covers lining)
- Channels or sluices to fill by gravity at high tide, or a means to lift water (Water Lifting)
- Rakes, baskets and drying racks
- A marked stick to measure depth
Method: Run A Salt Season
Step 1: Build and seal the ponds so that the largest is shallow and first and the last is small.
Step 2: Fill the first pond with seawater and let it concentrate.
Step 3: Move the brine from pond to pond as it concentrates, leaving gypsum and mud behind.
Step 4: In the final pond, let crystals form on the bottom. Rake them onto the bank to drain. Leave a layer of salt on the bed to keep the harvest off the mud.
Step 5: Drain off the bitterns before the pond dries, and dry the salt (Purification & Grading).
Common Mistakes
- Letting the last pond dry out. The bitter salts crystallise last and ruin the harvest.
- Scraping the pond bed. It puts mud and clay in the salt.
- Siting ponds below the high-tide line.
- Ignoring leaks. A pond that seeps loses the brine you worked to concentrate.
Safety
Sun, glare and standing water
Raking salt in full sun causes heat illness and, from the glare off the white crystals, eye damage: wear eye protection and a hat and carry drinking water. Brine in cuts stings and slows healing. Ponds, even shallow ones, are a drowning risk for small children and a trap for stock; fence them.
What’s Next
- Learn next: Yield calculations — what a season’s ponds can really produce
- Combine with: Boiling method for cloudy climates — the fallback in a wet year